A source who has spoken with Forte told NFL.com and NFL Network's Ian Rapoport the running back has suffered a high ankle sprain, but the Bears do not have a clear timetable for his return. Forte's availability will depend on his pain tolerance, the source said.

Not only does the Bears' offense start with the run, but Forte brings a sense of calm to complement the at-times-wild Jay Cutler. Bush becomes the starter if Forte is out for any length of time.

"That's why I'm here: They brought me here to pick up where (Forte) left off, if something went wrong," said Bush following Thursday's loss, according to the Chicago Tribune. "It's the insurance plan, I guess. I'll be ready."

Bush has been one of the premiere No. 2 running backs in the NFL. He ran for 977 yards in 2011, most after Oakland Raiders starter Darren McFadden was injured. Bush had 54 yards on 14 carries against the Packers.

Bush brings more of a power game to the backfield than the sure-handed, slick-stepping Forte. Though a different style of back, the Bears run game should be fine if Forte misses games.